Amyotrophic Lateral Sclerosis (ALS) is a lethal neurodegenerative disease that involves the selective loss of motor neurons in the brain and spinal cord. Post-mortem analyses of ALS patients reveal ubiquitinated protein aggregates in motor neurons. Currently, we do not know whether these aggregates are toxic, protective or merely an epiphenomenon of the disease. ALS-associated aggregates often stain positive for Sequestosome-1 (SQSTM1/p62), a ubiquitin binding protein that plays a role in proteasome and autophagy mediated degradation. While some other components of these aggregates are known, a complete proteomic characterization is not currently available. Furthermore, the role of p62 in aggregate formation and degradation is not well understood. The SOD1 G93A mouse model recapitulates many features of ALS, including the accumulation of p62-positive aggregates in motor neurons. To better understand the biological significance of these structures, I have optimized an approach for immunoprecipitating p62 from the SOD1 G93A spinal cord. Preliminary mass spectrometry analysis of these immunoprecipitates reveals that Valosin-containing protein (VCP) is present in p62-positive aggregates. I plan to build on this finding by using unbiased proteomics to identify other p62-interacting proteins. I have also discovered that p62 is phosphorylated in the SOD1 G93A spinal cord, and I will test how this post-translational modification regulates p62 function. Lastly, I will determine the role of p62 in vivo by breeding SOD1 G93A mice to p62 knockout mice and monitoring the disease phenotype.